An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions. An LCD generally includes a liquid crystal panel, a driving circuit for driving the liquid crystal panel, and a backlight module for illuminating the liquid crystal panel.
FIG. 2 is essentially an abbreviated circuit diagram of a typical LCD 100. The LCD 100 includes a liquid crystal panel (not shown), a gate driving circuit 110 and a data driving circuit 120. The gate driving circuit 110 and the data driving circuit 120 are formed on the liquid crystal panel by a chip on glass (COG) method. The gate driving circuit 110 is used to scan the liquid crystal panel. The data driving circuit 120 is used to provide gray-scale voltages to the liquid crystal panel when the liquid crystal panel is scanned.
The liquid crystal panel includes a pixel array 130 and a short-circuit test circuit 140. The pixel array 130 includes a number of gate lines 111 that are parallel to each other and that each extend along a first direction, and a number of data lines 121 that are parallel to each other and that each extend along a second direction orthogonal to the first direction. The gate lines 111 and data lines 121 cross each other, thereby defining an array of pixel units 150. The gate lines 111 are connected to the gate driving circuit 110. The data lines 121 are connected to the data driving circuit 120.
Each pixel unit 150 includes a thin film transistor (TFT) 151, a storage capacitor 152, and a common electrode 153. A gate electrode (not labeled), a source electrode (not labeled), and a drain electrode (not labeled) of the TFT 151 are connected to a corresponding gate line 111, a corresponding data line 121, and a terminal of the storage capacitor 152 respectively. The other terminal of the storage capacitor 152 is connected to the common electrode 153. The TFT 151 functions as a switching element for charging and discharging of the storage capacitor 152.
The short-circuit test circuit 140 includes a plurality of switching TFTs 141, a test control line 142, a first test lead 1401, a second test lead 1402, a third test lead 1403, a fourth test lead 1404, and a fifth test lead 1405. Each of odd-numbered gate lines 111 is connected to the third test lead 1403 via a drain electrode and a source electrode of a corresponding switching TFT 141. Each of even-numbered gate lines 111 is connected to the fourth test lead 1404 via a drain electrode and a source electrode of a corresponding switching TFT 141. Each of odd-numbered data lines 121 is connected to the first test lead 1401 via a source electrode and a drain electrode of a corresponding switching TFT 141. Each of even-numbered data lines 121 is connected to the second test lead 1402 via a source electrode and a drain electrode of a corresponding switching TFT 141. The fifth test lead 1405 is connected with gate electrodes of the switching TFTs 141 and finally the gate driving circuit 110 in series via the test control line 142. The structure of the short-circuit test circuit 140 is a so-called 2G2D structure.
The short-circuit test circuit 140 is generally used to test whether the gate lines 111 and the data lines 121 are damaged or not before the driving circuits 110, 120 are attached to the liquid crystal panel. When the liquid crystal panel is tested, each of the test leads 1401, 1402, 1403, 1404, 1405 receives a test signal. The fifth test lead 1405 provides a high-voltage signal to switch on the switching TFTs 141. The third test lead 1403 provides a high voltage to odd-numbered gate lines 111 to switch on odd-row TFTs 151. The fourth test lead 1404 provides a high voltage to even-numbered gate lines 111 to switch on even-row TFTs 151. The first and second test leads 1401, 1402 provide gray-scale voltages to the storage capacitors 152 respectively via odd-numbered data lines 121 and even-numbered data lines 121, thereby displaying test images on the liquid crystal panel. After the gate driving circuit 110 is attached onto the liquid crystal panel, the gate driving circuit 110 provides a low voltage to the gate electrodes of the switching transistors 141 via the test control line 142 so as to deactivate the short-circuit test circuit 140.
The LCD 100 is powered on by an external power supply (not shown), which connects with the LCD 100 via an external power supply connection of the LCD 100. After the LCD 100 is powered on, the gate driving circuit 110 provides a high voltage to the gate lines 111 so as to switch on the TFTs 151. The data driving circuit 120 provides a gray-scale voltage to the storage capacitors 152 via the data lines 121 and the activated TFTs 151. After being charged, the storage capacitors 152 each store a changeless amount of electric charge until a next gray-scale voltage is applied thereto.
When the LCD 100 is powered off, electric charge stored in the storage capacitors 152 generally cannot be discharged quickly. This makes the voltage at the external power supply connection drop slowly. As a result, the gate driving circuit 110 and the data driving circuit 120 operate incorrectly, thereby producing a residual image on the liquid crystal panel.
What is needed, therefore, is a new LCD that can overcome the above-described deficiencies.